Global Positioning Satellite (GPS) navigation as exemplified by NAVSTAR/GPS, is an accurate, three-dimensional navigation system which has become one of the most important technologies of the era, impacting a myriad of users from aircraft and ships, to farmers and hikers. The GPS comprises a constellation of twenty four satellites which orbit the earth twice a day. The orbits of the GPS satellites are maintained in a virtually circular manner at approximately 10,898 nautical miles above the earth, the GPS satellites orbiting the earth in six overlapping orbital planes based on the equatorial plane of the earth. These orbits are chosen so that the GPS system can provide information to users regardless of the time that the user requests information and regardless of the user's position on the earth's surface. This information contains a navigation message, which includes satellite ephemerides and satellite clock drift information.
In order for the system to operate properly, the orbits of the GPS satellites are maintained by a plurality of ground-based tracking stations. The ground-based tracking stations each use a standard GPS L-band transmitter/receiver to monitor and control the orbits of the GPS satellites. Each GPS satellite continuously broadcasts pseudo-random codes at L-band frequencies, L1 at 1575.42 MHZ and L2 at 1227.6 MHZ. One of these signals is referred to as C/A code, which is a signal that can be received by civilian type GPS receivers. The other signal is referred to as P code, which is a signal that can be received only by military type GPS receivers. The ground stations on the earth receive these L-band transmissions from the satellites. These transmissions are analyzed and GPS time is compared with universal standard time at the ground stations. Corrections are transmitted to receivers in each of the satellites from the ground stations.
A major benefit of the GPS is that the number of users is unlimited because the signals transmitted by the satellites are passively acquired. Thus, broad civilian and commercial applications are possible. For example, GPS navigation has been commonly applied in terrestrial (earth) based applications. In such applications, a GPS receiver can be located in mobile units such as ground vehicles to enable the vehicle operator to precisely locate his or her global position. GPS navigation has also been proven to be of value for aircraft and spacecraft use as well, with such "non-terrestrial" mobile units employing a GPS receiver for precisely locating the unit's global position.
The user's GPS receiver operates by engaging in a radio-ranging calculation which involves acquiring the encoded signals transmitted by each GPS satellite and making pseudorange measurements. These measurements are processed in real time to provide the best estimate of the user's position (latitude, longitude, and altitude), velocity, and system time. The user's receiver maintains a time reference which is used to generate a replica of the code transmitted by the satellite. The amount of time that the receiver must apply to correlate the replicated code with the satellite clock referenced code received from the satellite provides a measure of the signal propagation time between the satellite and the receiver. This time propagation or "pseudorange" measurement is the error by the amount of time synchronization error between the satellite and receiver clocks.
The user's receiver then, employs a three dimensional equivalent of the traditional "triangulation" technique on the data it receives from the GPS satellites to compute the user's position. In order to use this "triangular" technique, four of the orbiting GPS satellites must be "visible" to the user at any one time, and the position of these four satellites relative to the earth must be known.
As mentioned above, GPS navigation has been proven to be of value for spacecrafts such as communication satellites and the like. Traditionally, however, such satellites are located and positioned by multiple tracking stations located on the ground. A ground tracking station's fixed position relative to the earth makes it possible to accurately compute the position of an orbiting satellite relative to the tracking station. This is particularly important in satellite systems that cannot function properly without knowledge of satellite position relative to the earth's surface. Typical of such a system would be any communication network that proposes to provide global point-to-point coverage through the use of earth-orbiting satellites. In order for such a system to be operable, it must be capable of determining if the satellite selected to relay the communication is "visible" to both points on the earth, or if a satellite-to-satellite relay is required because the source and the destination are not visible from the same satellite. The visibility of the satellite to any point on the earth is a function of the position of the point, the position of the satellite, and the beamwidth of the satellite's antenna. Since the position of a point on the earth's surface and the beamwidth of the satellite's antenna are known, the only remaining unknown necessary to a visibility determination is the satellite position. The ground station provides a way to determine this satellite position.
For high orbit satellites, this method of satellite positioning is geometrically limited in the horizontal direction. Even with a long baseline between ground stations, location accuracy is typically 20-30 meters, RSS. GPS navigation, on the other hand, holds the promise for a low cost, and autonomous solution for high orbit spacecraft location with a minimum of a three times accuracy improvement. However, for spacecraft such as satellites, that are at orbits higher than the GPS satellites, the GPS L-band signal becomes unreliable at best. The cause of the problem is a lack of L-band signal visibility at these higher orbits. Referring to FIG. 1, it can be seen that only one or two GPS satellites are typically visible by a satellite or spacecraft at high orbit. As mentioned earlier, minimum of four are required for orbit determination.
Recently, however, a newer generation of GPS satellites referred to as GPS block IIR satellites have been provided with an autonomous navigation capability referred to as "AUTONAV", which enables the GPS satellite's position to be predicted up to 210 days instead of 14 days. The AUTONAV system was developed to reduce the GPS satellites' reliance on ground stations for navigation. The AUTONAV system employs a special transponder unit in each GPS Block IIR satellite which is designed to transmit ultra-high-frequency (UHF) signals between the GPS satellites for the purpose of intersatellite ranging. Unlike the L-band signals which are directed at the earth, UHF signals are visible to high orbit satellites and spacecraft. In FIG. 2, a graphical plot of visibility vs. time is shown which indicates that typically, eight to ten GPS satellites would be visible to satellites at geostationary orbit (a minimum of four is required). Geostationary orbits would be one of the more likely places for "high orbit" satellites. While this intersatellite data was not intended for use beyond the GPS constellation, clearly the UHF signals are visible at Geostationary orbits. Additionally, a radio frequency link analysis indicates a 7.1 dB typical signal margin for this case.
Accordingly, it is an object of the present invention, to provide a totally passive navigation technique and system for high orbit spacecraft such as satellites, which operates on UHF based Time intersatellite data transmitted between the GPS block IIR satellites.